This is something that I was thinking about in regards to the Federation and the Klingon Empire

In the plot of Deep Space Nine a major point of the series brought up time and time again was the occupation of Bajor by the Cardassian Union, a 60 year period in which the Cardassians strip mined the planet, oppressed the population and killed some 15 million Bajorans (a little bit on the low side compared to some of earth's civilizations, but it still is horrible). Many Bajorans became refugees scattered across the galaxy. This was made out as a horrible ordeal for the Bajorans that they are still recovering from and a major sign of why the Cardies are the Bad Guys. At the same time there is another faction which is of a similar quality, the Klingon Empire which is often depicted as the allies of the Federation. When not, it's generally shown to be due to the influence of the Dominion. Even so, there is an unpleasant level of hypocrisy that's in the subtext.

While the Klingons at this point were on good terms with the Federation for the better part of a century what can not be overlooked is their history. For about three centuries before Khitomer they were an expansionistic power, sending out starships to conquer other civilizations and ad them into their Empire. Needless to say this would mean a bunch of planets being conquered. We see refugees fleeing the Klingon Empire in Enterprise and the policy of Klingons. As to why this is the case remember that on Organia the policy of the Klingon Empire was to kill a thousand guys in reprisal for the Death of one dead Klingon soldier and that they were enslaving the population. Even after the Khitomer Accords put a stop to their expansion, that still would leave dozens of planets under the Klingon jackboot with little sign of reform in regards to their colonial subjects. The Klingon System of government requires one to prove himself as a warrior before he can hold office, but we never see a non Klingon serving as a warrior. This means that even by the standards of a non democratic feudal warrior society the non Klingon subjects of the Klingon Empire are disenfranchised and take a look at Medieval Europe or Feudal Japan and see what it would be like for the Serfs.

Imagine if you will a scene after the Dominion War in a Klingon System where an Akira class ship is getting a refit while the crew takes shore leave on the Station. Among the crew was the child of a Bajoran Refugee who had fought hard not only to protect her new home, but to liberate her ancestral home from the oppression of the Cardassians (again). Her and her crewmates soon meet up with some Klingon Warriors who they'd fought alongside and spend some time with each other. The Klingons actually went out of their way to have a good time with their comrades in arms have replicated a decent Korean BBQ, Plomeek Soup and Bajoran Ale to go along with the Blood Pies and Gagh they exchange tales of their experiences saving the Alpha Quadrant from Dominion domination while their ship gets spare parts. Said spare parts come from the planet below, in which twelve billion people are born, raised, eat, sleep and die in large industrial complexes, but mostly work. They sleep in huge barracks, eat ration slop in mess halls, walk through sonic showers like cattle and are led to their work stations where they spend twelve hours a day building starship components under the supervision of low ranking bitter warriors tasked with the most menial of jobs: keeping the workers in line. Those who slack off get zapped with pain sticks or get a beating from their overseers, those who lash out against them get put down like diseased dogs, or if they show any particular fight are put into pits full of starved targs to for the amusement of the guards and as an example to the other slaves. This is how it has been for them for more than two centuries and only a few half remembered tales speak of the world before the Klingon masters came.

That is speculation on the extreme side of things, BUT there are plenty of milder variations of this theme which are still pretty horrible and would be consistent with canon. Always remember kids, enslavement by Reptilian Fascists is bad but enslavement by lumpy headed feudal barbarians is A-OK!

An alternate take on all this is that the Klingons abandoned a lot of these colonial possessions during the post-Praxis era when they needed to concentrate on restoring the ecosystem and infrastructure of their own homeworld rather than having millions of warriors scattered across dozens of planets occupying restive local populations.

Or that the Klingon presence on vassal worlds became so light that they barely notice the presence of the Klingons themselves as long as they pay their taxes... Because real feudal systems generally work by co-opting the local rulers and giving them a title within the feudal system, or at most deposing the topmost layer of the government and replacing it with your own relatives.

Furthermore, it is very much unclear whether the Klingon warrior class that wages war is actually all that large, as opposed to being something more like historical Norse societies where the vast majority of the population is off doing economically productive things while only a small percentage actually "goes a-viking" and travels out in ships to raid or trade or fight as mercenaries at any one time. In which case, well, they won't be brutally oppressing tens of billions of serfs on planets any more than the Norse were able to brutally oppress tens of millions of serfs in Europe. Sure, Norse adventurers could move into an area and take it over (e.g. Normandy or the Kievan Rus), but they invariably wound up so heavily assimilated that they couldn't run an extractive economy for the benefit of the Scandinavian homeland. They just wound up identifying with the locals as a rulership class.

...

Bluntly, we really don't get enough of a look at the internal dynamics of the Klingon Empire to draw firm conclusions. Trying to extrapolate from a few specific incidents around how they were portrayed in the original series is just a way of randomly stirring up shit.

The Klingon's seem like the kind of origination where what would happen over that kind of timescale, four centuries, is that the Klingon occupiers would all be pretty well integrated into the occupied societies, perhaps even the local populations are even allowed armament, just not service in the star navy. But that doesn't all the Klingon themselves are actually well unified. In fact it seems like rule in the Empire is entirely on the basis of personally commanding a star fleet of sufficient proportional strength to enforce power, and basically summoning that up exactly like a damn feudal levy if need be.

So the people on the occupied planets are probably big time pawns in that kind of game, and local treatment would vary accordingly. A lot of them are probably just kept content in various industries that slowly and inefficiently but surely grind out the parts for more warships. And this leaves all the damn Klingon's to brash about and constantly posture instead while whoever happens to rule the Empire at the moment tries to ensure some damn ships are actually patrolling the borders, and constantly attacking other powers so none of them know how dis unified Klingondom actually is.

Abandoning some colonies after Praxis does sound likely, though you'd also figure after the Klingons recovered from Space Chernobyle they'd probably try to reassert rule over some areas too. Places they might evacuate, but refuse to let out of protectorate treaties ect.

"This cult of special forces is as sensible as to form a Royal Corps of Tree Climbers and say that no soldier who does not wear its green hat with a bunch of oak leaves stuck in it should be expected to climb a tree"
— Field Marshal William Slim 1956